It is the purpose of the proposed research to determine the nature, causes, and effects of the incorporation of chromosomal rearrangements in natural populations of mammals. Cytogenetic and population genetic data will be collected and correlated with biogeographical, ecological, and population structural data for selected natural populations of Peromyscus. In this genus, as in man, phenotypically normal individuals are characterized by constant diploid number, but may possess pericentric inversion and heterochromatin polymorphisms. Chromsome banding analyses will be used to identify chromosomal rearrangements and to determine the frequency and geographic distribution of the rearrangements. Population genetic data will be obtained for cytogenetically characterized populations using electrophoretic analyses to determine levels of genetic polymorphism and divergence. These data will be used to characterize population structure as to intra and interpopulational gene flow. Cytogenetic and population genetic data will be analyzed for correlations between type and amount of chromosome rearrangement and effective deme size as determined by degree of inbreeding. Effects of chromosome rearrangements upon gene flow between conspecific populations will also be assayed. It is hypothesized that chromosomal rearrangements may provide a selective advantage and that incorporation and subsequent fixation of such rearrangements can be a function of selection resulting in the establishment of a more fit karyotype. Small effective deme size and inbreeding therefore would not be a necessary prerequisite for phyletic chromosome reorganization. This hypothesis has important implications with respect to the potential for chromosomal evolution in man and other mammals which are not generally characterized by populations subdivided into small, isolated demes.